1. Field of the Invention
The present invention relates generally to bridge and fracture plugs used in oil and gas wells.
2. Related Art
Just prior to beginning “production,” oil and gas wells are completed using a complex process involving explosive charges and high pressure fluids. Once drilling is complete, a well is lined with steel pipe backed with cement that bridges the gap between the pipe outer diameter and rock face. The steel/cement barrier is then perforated with explosive shaped charges. High pressure fluids and proppants (spherical sand or synthetic ceramic beads) are then pumped down the well, through the perforations and into the rock formation to prepare the rock for the flow of gas and oil into the casing and up the well. This fracturing process is repeated several times in a given well depending on numerous factors including the depth of the well, casing diameter, reservoir pressure, the number of oil or gas bearing layers, etc.
The number of layers to be perforated and fractured can be as few as one or more than thirty. As they prepare to “frac” (i.e. hydraulic fracturing) at each level, well technicians set a “temporary plug” in the bore of the steel casing pipe (just below where they will perforate) that will then allow them to pump “frac fluids” and sand down through the perforations and into the oil and gas bearing layers of rock. Use of the temporary plug prevents contaminating the already-fractured levels below. This process is repeated several times, as the frac operation moves up the well, until all desired zones have been perforated, fractured and the needed amount of proppant has been pumped into the rock. At each level, the temporary plugs are usually left in place, so that they can all be drilled out at the end of the process, in a single operation.
These “temporary plugs” have traditionally been made from cast iron. These cast iron plugs have a threaded center mandrel and a threaded locking ring set inside of a threaded push sleeve. When the plug is set, a setting sleeve pushes against the top of the push sleeve and compresses the stack of slips, cones and rubber elements. The rubber elements expand outward and inward and create a seal between the elements and mandrel and the elements and the inner diameter of the well casing. The lock ring engages the threads in the mandrel and the threads in the push sleeve to prevent backward (i.e. upward) movement once the force from the setting tool is released. This locking action keeps pressure on the elements which preserves the seal and keeps the slips locked to the inner diameter of the casing. This blocks fluid from getting to the lower layers of rock and creates the seal needed to perform hydraulic fracturing in the layers above the plug.
It has been proposed to make plugs from other materials, such as aluminum alloy, which can use a push sleeve, locking ring and threaded mandrel similar to that described above. It has also been proposed to make plugs from composite materials. Some composite plugs can use a push sleeve to retain a locking ring that bites into the composite mandrel directly to keep the elements compressed and the slips locked in place after the setting force is removed. Other composite plugs have a fixed top stop so that the upper structural stop does not translate axially like a push sleeve, but rather stays fixed in place. Once the slips are locked to the casing inner diameter and the elements are compressed, the mandrel appears to be free to slide up and down in the elements (or stroke) until the top stop or anvil contacts the upper or lower slips. The upward movement can be caused when pressure from the oil or gas below exceeds the pressure applied from above. The downward movement acts in the opposite manner. Some elements or packers have directional properties. For example, some elements or packers have a greater upper seal pressure than lower seal pressure. Allowing the mandrel to stroke can change the relative motion and change the directional properties of the seals.
Examples of such plugs include U.S. Pat. Nos. 3,306,366; 3,517,742; 4,708,202; 5,131,468; 5,224,540; 5,701,595; 6,167,963; 6,220,349; 6,354,372; 6,581,681; and US 2004-0036225 and 2005-0189103.